One method of welding low-alloy steel or a similar steel material is a method that uses a high-density electron beam (see PTL 1). It is known that the crystal grains become coarser and the toughness deteriorates in weld zone. Accordingly, the weld is subjected to a post weld heat treatment or the like to restore the toughness.
Further, in steam generators of pressurized water reactors and the like, the desire to increase output of power generation plants has lead to demands for larger generators, and this has been accompanied by a trend toward the use of thicker plates in structures.